Radio communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using radio-frequency transmitters. To communicate such information, radio communications systems employ radio transmitters. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal.
A common problem in radio transmitters, radio-frequency amplifiers, and other electronic devices is non-linearity of signal gain. Non-linearity may cause amplifier gain to be dependent upon input signal amplitude and as a result may cause harmonic distortion and other undesired effects. Of particular concern is third-order non-linearity which is in many cases the dominant type of non-linearity, resulting in a phenomenon known as third-order intermodulation. In a radio transmitter, harmonics may be introduced from both a baseband signal and a local oscillator used to modulate the baseband signal. Such harmonics can impact the performance of a transmitter in at least two ways, both of which can generate signal components outside of an allowed spectral mask, and thereby, may cause spectral interference to other devices.
The first mechanism is the intermodulation between harmonics of the local oscillator and baseband signals, know as counter-intermodulation. The second mechanism is the harmonic distortion of an upconverted radio-frequency signal by the radio-frequency amplifier of the transmitter. Modulators may be designed to reduce counter-intermodulation. However, non-linearties in radio-frequency amplifiers may cause regeneration of the counter-intermodulation.
Traditional approaches to solving the above problems have disadvantages. For example, a technique known as inductive degeneration is often applied in radio-frequency circuits to improve circuit linearity. However, this technique does not address the problem of counter intermodulation. As another example, a technique known as derivative superposition may be used to cancel third-order non-linearity. However, derivative superposition is not effective in reducing counter-intermodulation regeneration. As a further example, an output of the modulator of a transmitter may be high-pass filtered to reduce counter-intermodulation, but such approach does not eliminate non-linearities of the amplifier which may regenerate the counter-intermodulation.